The number of persons developing dementia or Alzheimer's disease at some point in their lives is enormous and growing with our aging population. As described by Evans et al. (1989), "Alzheimer's disease is a common condition and its public health impact will continue to increase with the increasing longevity of the population." An estimated 10.3% of the population over age 65 has dementia of the Alzheimer's type ("DAT") (Evans et al., 1989). The prevalence of DAT in persons over the age of 85 ranges from about 20% (Selkoe et al., 1992) to over 50% (Evans et al., 1989).
The economic and social impact of DAT also is enormous. The estimated direct cost in the United States for treating DAT in 1991 was $20.6 billion. (Ernst and Hay, 1994.) Assuming that the prevalence of DAT remains constant in the future, a conservative estimate of the economic impact of DAT on the next generation, in discounted present value, is $536 billion for direct costs and $1.75 trillion for both direct and indirect costs. (Ernst and Hay, 1994.)
No easily administered, non-invasive, sensitive and specific test for screening a person for DAT and evaluating his or her probability of having DAT exists today. Such a test could prevent at least some of the disease's adverse economic and social consequences. The potential market for such a test includes neurologists, geriatric psychiatrists, neuropsychologists, gerontologists, primary care physicians, ophthalmologists, optometrists and other health care workers. Clinicians, researchers and pharmaceutical companies also could benefit from an easily administered and inexpensive test for identifying the effectiveness of treatments for DAT. For instance, patients on tacrine (Cognex), donepezil (Aricept) and other cholinergic enhancing medications could be monitored for evaluating the effectiveness of such treatments.
The current tests for evaluating the probability of a person having DAT are expensive and invasive. For instance, single photon computed tomography (SPECT) and positron emission tomography (PET) are costly, require expensive equipment and require the injection of radioactive substances into the body. Such techniques are inappropriate for screening large numbers of persons. Another test includes screening for APOE-I4 allele which is associated with an increased risk for DAT. Possession of APOE-I4 allele, however, does not guarantee the presence of DAT, and the absence of APOE-I4 allele does not rule out DAT. Between 23% and 57% of persons having DAT would be misclassified if possession of APOE-I4 allele alone were the criterion for diagnosis.
Another test for evaluating the probability of a person having DAT detects the presence of neural thread protein (NTP) from cerebrospinal fluid (CSF). This test claims to be the "first test proven to help physicians be certain in the diagnosis of Alzheimer's disease." This test, however, also is invasive and requires a spinal puncture to obtain a CSF sample.